A number of current techniques are employed to measure various characteristic of a signal generator (or device under test (DUT)) for testing and analysis purposes. The current techniques involve generating signal generator characteristics using linear measurements and models only, and therefore provide no non-linear describing behavior, even though signal generators contain active components, such as amplifiers, that exhibit non-linear behavior.
Typically, measurements performed on signal generators involve use of a power sensor and vector network analyzer (VNA) configuration. The power sensor is used to measure output signal characteristics of the signal generator (or device under test (DUT)), and the VNA is used to measure output impedance of the signal generator. However, there are a number of disadvantages of this approach. For example, the VNA is used to measure impedance match of the output port(s) of the signal generator only when the signal generator is not generating an output signal, resulting in output impedance measurement constraint. The impedance of the output port(s) of the signal generator is typically different when comparing a signal “on” state versus a signal “off” state. The more accurate measurement is obtained when the signal generator is “on” and generating a signal, but such a measurement is not possible with the standard VNA approach. Also, power sensors are broadband receivers, so any non-linear distortion (such as harmonics) of the output signal is measured by the power sensor in combination with the fundamental signal. Components of the fundamental frequency and components of the distortion cannot be distinguished from each other, resulting in errors in measured results when trying to measure a signal at a desired fundamental frequency. In addition, the dynamic range the power sensor is low compared to a VNA tuned-receiver, the power sensor impedance is imperfect (e.g., 50 ohms), thus affecting performance of the signal generator when connected, and the power sensor provides amplitude but no phase information. The power sensor also has low linearity compared to the VNA tuned-receiver.
In another conventional approach using a linear model, a VNA utilizes offset frequency hot-match measurements, where the VNA measures amplitude and phase of the output signal from the signal generator, as well as a “hot” S-parameter to determine output impedance of the signal generator. However, according to this approach, a signal source and receivers of the VNA are offset in frequency relative to the signal generator, thus constraining the output impedance measurement. This is because an error corrected S-parameter impedance match measurement is made on the signal generator while the signal generator is generating an output signal, and the offset in frequency enables the ability to distinguish between the signal generated by the signal generator and the signal generated by the VNA signal source. Ideally, the signal generator characteristics should be identified at the same frequency of the generated signal, and not offset in frequency, which introduces errors in the measured results. Another limitation is that this approach assumes a linear model and therefore describes only the linear behavior of the signal generator.
In yet another conventional approach, a VNA implements a coupler and open/short ripple technique. Again, this uses a linear model, and thus no non-linear describing behaving is obtained. Also, there is high uncertainty, as the technique relies on the assumption that the signal generator operation is not influenced by high reflected signals. Also, a power sensor is required for measurements of amplitude (or power), as discussed above.
Accordingly, the current linear methodologies are insufficient, and a non-linear measurement and modeling approach is needed.